Since the introduction of plastics in the 1940's after World War II, engineers have been utilizing plastic to make any number of different articles. The plastic industry started with an injection molding machine and method, and has since been advanced into blow-molding, roto-molding, compression-molding, and other means of making plastic articles. Making a mold for an injection molding machine, which is the most common form of forming plastic, is basically a series of steps which include the manufacture of an aluminum or Kirksite mold in order to produce proto-type plastic parts. These molds generally can produce a number of plastic articles on the order of from approximately 100 parts to about 2 to 3 thousands parts, depending on what the tolerances are for the molded articles themselves. If the molded articles have a lot of sharp comers and high tolerance components, the aluminum or Kirksite mold will quickly degrade in quality and maintain its tolerances due to the fact that high pressure is utilized to force melted plastic particles into the mold with such pressure that it will force the plastic into all the nooks and crannies of the mold, thereby forming a complex part. As injection molding is the most common of the plastic forming techniques, it is well known to be the least expensive means for forming large numbers of such plastic articles.
Although injection molding is the least expensive process, this is only the case if the plastic articles being formed are being made in large volumes. The cost of the injection mold must be amortized over the cost of all of the plastic parts. For instance, if a mold costs $150,000 U.S. dollars, and it is planned to form 600,000 parts, then $150,000 or one-quarter, of the costs of each of the 600,000 parts must be attributed to the cost of the mold. Therefore, it has always been an expensive proposition to run a small number of plastic parts with an expensive injection mold. The cost of the mold may raise the cost of the plastic parts to a point where it is not feasible.
Other methods of forming plastic have their advantages and disadvantages as well. Some of them do not produce parts that have smooth surfaces on both sides, some of them produce articles utilizing even more expensive materials and producing parts that will not crack and stress, while other methods may produce one-sided articles, which may or may not be suitable for certain production items.
Previous methods have formed plastic articles that have modulus strengths which are limited by the modulus strengths of the plastic itself. A steel article has a modulus strength on the order of a 30 modulus. Other materials have modulus strengths generally therebetween. Prior art methods have not been known to strengthen the modulus strength of certain plastic articles, with the exception of the inclusion of various chopped fibers and the like. In addition, load bearing structural elements have been produced for large, self-supporting panels having a high weight to strength ratio for bridge components, airplane decks, and the like. These components have formed roof panels and the like although there are certain drawbacks which prevent it from being used in the automotive or aircraft industries. These components, however, generally have been produced with a poor surface finish, and will not provide the type of detail which may be necessary for many applications.
In addition, prior art methods have been utilized to incorporate a “sandwiched” form, including a foamed polyurethane core, which may be in the form of a foam sponge, or it may be a foamed core which produces gaseous substances during the curing, in order to increase the volume of the foamed core. Certain reinforcing fibers may also be incorporated in an attempt to increase the strength of the plastic.
By way of explanation of various other methods and embodiments of the method and resulting articles, we are hereby also incorporating by reference the U.S. Provisional Patent Applications No. 60/270,321 filed on Feb. 5, 2001; No. 60/300,874 filed on Jun. 25, 2001; No. 60/346,336 filed on Jan. 7, 2002; and International Application No. PCT/US02/03298 filed on Feb. 5, 2002.
Therefore, it would be a great advantage to the industry if a new method and apparatus could be promoted for inexpensively manufacturing large, strong plastic articles. It would be an even greater advantage if the molds were so inexpensive that small production numbers could become economically feasible.